Help adding new Subscriptions

I would like to add some new subscriptions, but I can’t find much detail on this in the docs, so could you please help with the following problem i have.

I want to create a new membership, one that has a higher subscription price, same levels. Simple enough for new subscribers, but how can I handle upgraders and what they pay to upgrade?

By this, I mean what happens if a user that signed up to the lower subscription almost a year ago, and wants to upgrade, but it’s not worth upgrading if they have to pay the full year fee.

E.g.

2 yearly subscriptions:

Subscription 1 = $50

Subscription 2 = $100

User subscribed to 1 6 months ago. So instead of paying $50 to upgrade now, could he pay $25 (%50 of upgrade price for %50 of year)

Thanks!